Field of the Invention
The present invention is in the technical field of energy storage devices. More particularly, the present invention is in the technical field of rechargeable batteries employing an iron electrode, and the separators used in those batteries.
State of the Art
The nickel iron (Ni—Fe) battery was independently developed by Edison in the United States and by Junger in Sweden in 1901. It was industrially important from its introduction until the 1970's when batteries with superior specific energy and energy density replaced Ni—Fe batteries in many applications.
However, Ni—Fe batteries have many advantages over other battery chemistries. The Ni—Fe battery is a very robust battery which is very tolerant of abuse such as overcharge and overdischarge and can have a very long life. It is often used in backup situations where it can be continuously trickle-charged and last more than 20 years. Additionally, the active material iron is much less expensive than active materials used in other alkaline battery systems such as NiMH or in non-aqueous batteries such as Li Ion. However, the low specific energy, low energy density, and poor power have limited the applications of this battery system.
The Ni—Fe battery is a rechargeable battery having a nickel(III) oxy-hydroxide positive electrode and an iron negative electrode, with an alkaline electrolyte such as potassium hydroxide. The overall cell reaction can be written as:2NiOOH+Fe+2H2O2Ni(OH)2+Fe(OH)2  (1)
The ability of these batteries to survive frequent cycling is due to the low solubility of the reactants in the electrolyte. The formation of metallic iron during charge is slow due to the low solubility of the reaction product ferrous hydroxide. While the slow formation of iron crystals preserves the electrodes, it also limits the high rate performance. Ni—Fe cells are typically charged galvanostatically and should not be charged from a constant voltage supply since they can be damaged by thermal runaway. Thermal runaway occurs due to a drop in cell voltage as gassing begins due to overcharge, raising the cell temperature, increasing current draw from a constant potential source, further increasing the gassing rate and temperature.
As shown in Equation (1), the overall cell reaction does not involve the battery electrolyte; however, alkaline conditions are required for the individual electrode reactions. Therefore, iron-based batteries such as Ni—Fe, Fe-air, and Fe—MnO2 batteries all employ a strong alkaline electrolyte typically of KOH, typically in the range of 30-32% KOH. KOH is typically employed due to its higher conductivity and low freezing point. LiOH may be added in cells subject to high temperatures due to its stabilization effects on the nickel electrode, improving its charge acceptance at elevated temperatures.
The separators used in the cell construction depends upon the types of electrodes used. In Ni—Fe cells with a pocket plate electrodes, the anode and cathode are kept electrically isolated using a spacer or a grid-like mesh inlay and are typically held in a rigid frame. However, the construction of these cells is more expensive as the electrode design is not amenable to lower-cost manufacturing methods. Furthermore, the large inter-electrode spacing of these batteries imposed by the rigid support limits high rate performance.
Cells constructed with plastic-bonded, sintered, fiber, or foam electrodes are often lower in cost than cells with pocket plate electrodes. The electrode manufacturing process is cheaper, easier, and provides greater consistency between electrodes than the pocket plate design. Unlike Ni—Fe cells with a pocket plate electrodes, these cells use microporous separators. These cells have other advantages besides lower cost such as higher rate capability and greater energy density since microporous separators are much thinner which keeps the inter-electrode spacing small as the electrodes are held in place through compression. Microporous separators that consist of polypropylene, polyethylene, and polyolefin blends have been used in alkaline battery systems. The non-polar nature of the polyolefin chain makes it difficult to wet the separator. This problem has been addressed by modifying the surface properties of the polyolefin materials used to form polymeric sheets, by graft-copolymerizing to those surfaces a monomeric substance which, after copolymerization, confers hydrophilic properties and, in some cases, ion exchange properties.
However, it has been observed that a failure mechanism for Ni—Fe cells appears to be iron or iron oxide accumulation on the separator. The accumulation can lead to an electrical short between the anode and cathode. Such an accumulation of iron or iron oxide leads to a premature failure of the cell.
An object of this invention is to improve the cell life of a battery comprising an iron electrode. Another object is to provide a separator/electrolyte combination that overcomes the problems of premature failure of a Ni—Fe cell. Providing such a solution would be of great value to the industry.